The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for the machining of cast iron parts by turning.
Cast iron materials may be divided into two main categories, grey cast iron and nodular cast iron. Cast iron materials typically often have an outer layer of cast skin, which may contain various inclusions of sand, rust and other impurities and also a surface zone which is decarburized and contains a larger amount of ferrite than the rest of the material.
The wear when machining grey cast iron materials with Al.sub.2 O.sub.3 -coated cutting tools is dominated by chemical, abrasive and so-called adhesive wear. In order to protect the cutting tool against chemical wear, it is desirable to use as thick Al.sub.2 O.sub.3 layers as possible. This is contradicted by the properties regarding adhesive wear that this type of layer generally possesses. Adhesive wear occurs when fragments or individual grains of the layer are pulled away from the cutting edge by the work piece chip formed. The surface zone with high amounts of ferrite particularly puts severe demands on the adhesive properties of the coating and in combination with the inclusions in the cast skin on the work piece, causes notch wear at the depth of cut on the main cutting edge.
Another feature in the machining of grey cast iron is its sensitivity to excessive amounts of Co binder phase in the interface between the cemented carbide cutting insert and the coating. Excessive amounts of Co binder phase deteriorate the adhesion between coating and cemented carbide and lead to flaking of the coating during machining.
Swedish Application 9502640-7 (which corresponds to U.S. Ser. No. 08/675,034, our reference 024444-233) discloses a coated cutting insert tool of a cemented carbide body of a composition 5-11 weight % Co, &lt;10 weight %, preferably 1.5-7.5 weight %, cubic carbides of the metals Ti, Ta and/or Nb and balance WC, especially suited for machining of low alloyed steel components by turning.